1. Field of the Invention
The invention relates generally to a ball screw unit, and more specifically to a ball screw unit in which a screw shaft does not rotate but moves linearly in its axial direction.
2. Description of the Related Art
A ball screw unit that includes a screw shaft and a ball screw nut that is screwed onto the screw shaft via balls is commonly used for an electric actuator or a shock absorber. For example, Japanese Patent Application Publication No. 2005-264992 (JP-A-2005-264992) describes that a ball screw unit, which has a ball screw nut that is connected to a motor and a screw shaft that moves linearly, is applied to a shock absorber.
A shock absorber is usually provided with a stopper so that a screw shaft is prevented from moving by an amount equal to or larger than a predetermined amount when a large external force is applied. Immediately before the screw shaft is stopped by the stopper, the screw shaft is moving at a high speed and a ball screw nut is rotating at a high speed. Therefore, a large rotational inertia force may be generated at the ball screw nut due to a sudden stop of the screw shaft, and an indentation may be formed in a path portion of a ball screw.